1. Field of the Invention
The present invention relates to integrated circuits and, in particular, to charge pump circuits used in integrated circuits.
2. Description of the Related Art
In some integrated circuits, it is desirable to have a circuit that provides a relatively high negative voltage. This is particularly true for integrated circuits that include memory devices, such as, for example, electrically erasable programmable read-only-memory (EEPROM) devices. The high negative voltage is applied to control gates of memory cells during erasure to erase the data stored in the memory cells.
Disadvantageously, many conventional circuits suffer from significant body-effect, which modulates a transistor threshold voltage when the voltage between the bulk and the source is not zero. For example, the body effect can cause the effective threshold voltages of the capacitor-connected MOSFET transistors in one or more pump stages to increase. Because the clock signals have small amplitudes, the body effect can cause transistor conductance to decrease. As transistor conduction decreases, the affected pump stages can become more resistive, which can disadvantageously limit current and adversely affect the charge pump efficiency.
While attempts have been made to reduce the body effect, using certain techniques, the gate of pre-charge MOSFET is insufficiently negative enough, and the pre-charge efficiency is relatively low.